The invention relates to a heated oxygen sensor diagnostic routine and, more particularly to a diagnostic routine performed to ascertain whether an automotive oxygen sensor is still suitable for use.
Oxygen sensors are employed in most modern internal combustion engines for monitoring the by-products of combustion in order to regulate the fuel-air mixture. A properly regulated air fuel mixture is necessary to achieve clean burning of the fuel. Achieving a clean burn is especially important in an automotive engine, where strict emissions standards are difficult to meet if the fuel is not cleanly burned.
The sensor generates an output voltage depending on the content of oxygen in the fuel-air mixture at the exhaust. If the exhaust gas is rich in oxygen, the sensor will produce a low voltage, close to zero volts. If the exhaust gas is rich in fuel, the sensor will produce a voltage close to one volt.
The output voltage and internal resistance of the sensor will also vary with the temperature and the age of the sensor. The internal resistance can vary from about 100 ohms to several million ohms depending on the temperature of the sensor. A cold sensor has a very high internal resistance, which drastically decreases once the sensor reaches an operating temperature of about 300 degrees Celsius.
Because of its high internal resistance when cold, the sensor is lean in O.sub.2 or unreliable during the engine warm-up phase. During this period the engine operates in an open loop where data from the sensor is not used to regulate the fuel mixture. Since it is highly desirable to reach a closed loop condition rapidly, where data from the sensor is used to regulate the fuel mixture, a heater may be included in the automotive oxygen sensor as shown in U.S. Pat. No. 4,938,194 to Kato et. al. The heater brings the sensor to its operating temperature faster than it would if heated by the engine exhaust gases alone. Thus, the heater allows the engine to reach closed loop operation more rapidly.
The oxygen sensor becomes less reliable with age, because physical wear and chemical contamination affect the output voltage and internal resistance of the sensor. With a failed oxygen sensor the engine will run inefficiently, taking a serious toll on the performance of the car.
In addition, operating an automobile with a failed oxygen sensor can have a significant environmental impact. The amount of air pollutants produced by the automobile will increase directly due to an unclean burn, and also indirectly due to the failure of the catalytic converter when it receives large quantities of unburned fuel. Furthermore, a failed sensor can increase fuel consumption, turning a normally efficient fuel consuming car into a gas guzzler.
When the sensor fails, the automobile owner typically does not suspect the oxygen sensor, or may not even be aware that the automobile has an oxygen sensor. In addition to the possible expense incurred when the car fails to meet optimal emission levels, the owner is put to the unnecessary expense of having a mechanic troubleshoot the car to determine that the sensor has in fact failed. Often unnecessary work will be performed by an inexperienced mechanic who does not know to check the oxygen sensor heater to see if it has failed.
When the heater fails, closed loop operation cannot be achieved until the oxygen sensor has been heated sufficiently by the engine exhaust to the sensor's optimal operating temperature. Depending on the driving conditions, this time period can vary tremendously. While the oxygen sensor is warming up, and the engine is in open loop operation, the engine will perform inefficiently, since the fuel-air mixture will be less than optimum and may not meet today's emissions standards. It is therefore highly desirable to know whether the heater is still functional.
U.S. Pat. No. 4,742,808 to Blumel ('808) discloses a means and method for measuring the internal resistance of an oxygen sensor using two resistors. These resistors are alternately switched into the measuring circuit to obtain reliable measurements--reliable in the sense that two measurements are better than one. The test in the '808 patent determines if the sensor is ready for closed loop operation and does not detect wear or inoperability of the sensor. Further, the two resistors used in the '808 patent are not indicated as being of substantially different impedances, which would allow one to be used to quickly detect changes in the oxygen sensor internal impedance.
U.S. Pat. No. 4,844,038 to Yamato et. al. ('038) discloses a method for determining the deterioration of oxygen concentration sensors. The '038 patent diagnoses a sensor as abnormal if the output signal of the sensor remains substantially constant for a predetermined length of time. Thus, it does not provide a means for quickly detecting changes in sensor impedance.